The present invention relates generally to voltage-controlled oscillators, and more particularly to methods and devices for LC voltage-controlled oscillators.
Oscillators are widely used to provide clock signals in digital systems. In many applications the frequency or phase of the clock signal may require adjustment. An example of such an application is a receiving unit in a digital data communication system. The transmission of data in digital form is widely performed. An important consideration in the transfer of digital data is the speed at which data is transferred. Including a clock signal with data being transferred reduces the bandwidth available for transmission of data. Accordingly, in order to recover transmitted data, receiving units often require a means of synchronizing an internal clock of the receiving unit to the frequency and phase of the received data.
Controllable oscillators are often used to provide a clock signal for the receiving unit. Controllable oscillators, such as voltage controlled oscillators (VCOs), are adjustable in both frequency and phase, and therefore may be conforming to the frequency and phase of the received data. Consequently, voltage controlled oscillators are widely used in communication systems, particularly in applications such as phase locked loops (PLLs) and clock recovery circuits (CRCs).
These communication systems often use monolithic integrated circuits. Monolithic integrated circuits, however, are subject to process variations, such as temperature variations, which affect circuit components. The process variations affect the performance, including the tuning frequency, of oscillators incorporated on chip.
One type of VCO which may provide clock signals at a range of frequencies is an LC oscillator using diode varactors. Diode varactors provide an adjustable capacitance, which enables adjustment of the oscillator frequency. LC oscillators using diode varactors, however, are not without problems. For example, diode varactors sometimes have a limited linear voltage-to-current range, which increases design difficulties if the linear range is not sufficiently large for desired purposes. Further, process variations, such as temperature variations, in forming diode varactors in integrated circuits may result in less than desired VCO performance.
A voltage-controlled LC-tuned oscillator based on interpolating two different delay paths has been reported in N. Nguyen and R. Meyer, "A 1.8 GHz Monolithic LC Voltage-Controlled Oscillator," IEEE J. Solid-State Circuits, Vol. 27, No. 3, pp. 444, March 1992, which is hereby incorporated by reference for all purposes. This voltage-controlled LC-tuned oscillator, however has a structural conflict between the VCO tuning range and the resonator Q value.
A ring oscillator is sometimes used in a VCO. A ring oscillator is a ring of delay cells whereby the oscillator frequency is changed by changing the delay of each delay cell. Ring oscillators may provide extended tuning range, but ring oscillators also often have high phase noise since each stage of a ring oscillator introduces noise into the system due to a lack of a high Q element in the delay cells.